1. Field of the Invention
A technical field of the present invention relates to a method for manufacturing a semiconductor substrate.
2. Description of the Related Art
It is known that silicon carbide as a semiconductor material is more advantageous than silicon in terms of increasing the withstand voltage of a semiconductor element (for example, a transistor), reducing a loss of electric power, or the like. Therefore, it is expected that silicon carbide is used for practical application of a transistor for electric power.
The cost of a silicon carbide substrate itself is the biggest problem for realizing the semiconductor element using silicon carbide. It is difficult to melt silicon carbide because of its characteristics; therefore, the silicon carbide substrate cannot be manufactured by a Czochralski (CZ) method or the like which are used for manufacturing a silicon substrate. Therefore, a sublimation recrystallization method which is disadvantageous in terms of productivity has to be used for manufacturing a silicon carbide substrate for application of semiconductor (for example, see Patent Document 1 and Patent Document 2). The sublimation recrystallization method is a method in which a material is sublimated by heating and single-crystal silicon carbide is grown on seed crystal; therefore, there are problems in that a device in which the sublimation recrystallization method is performed at a very high temperature of 2000° C. to 3000° C. is needed and it is difficult to increase an area of the silicon carbide substrate.
Moreover, there is also a problem in that the silicon carbide substrate has defects called micropipes. The micropipes are hollow-core defects with a diameter of about 1 μm to 3 μm. If the micropipes exist in a semiconductor element, conductive defect occurs locally, and as a result, operation defect of a semiconductor element occurs. Other than the micropipes, there are also problems such as dislocation or the like.
After all, although a semiconductor element having a silicon carbide is expected to be high performance device, its commercialization is delayed in practice due to low productivity and low quality of crystals.
[Reference]
[Patent Document 1] Japanese Published Patent Application No. S62-66000
[Patent Document 2] Japanese Published Patent Application No. H2-48495